A spectroscopic instrument is an instrument for measuring absorbency of a testing sample by irradiating light from a light source onto the test sample and changing intensity of the light transmitted through or reflected from the test sample to an electric signal, and has been applied to various fields. When a spectroscopic instrument is applied to, for example, a color measurement or a blood test, out of light transmitted through a test sample, absorbency of a plurality of light components different in wavelengths, namely, each of the multiple wavelengths is measured. Such spectroscopic instruments include a rotary plate-system spectroscopic instrument as disclosed in Japanese Unexamined Patent Publication No. Sho-59-131124, for example. In this spectroscopic instrument, by mechanically rotating a rotary plate so that a filter which transmits a light component of a wavelength to be detected is located in an optical path, detection of multiple wavelengths is enabled.
However, since the rotary plate-system spectroscopic instrument selects a filter by mechanically rotating the rotary plate, it takes time to detect multiple wavelengths. Although a quick examination of multiple samples and multiple items is demanded in a blood test, the rotary plate-system instrument cannot satisfy this demand.
Instruments to satisfy this demand include half mirror-system spectroscopic instruments as disclosed in Japanese Unexamined Patent Publication No. Hei-11-6766 and Japanese Unexamined Patent Publication No. Sho-59-170734, for example. These spectroscopic instruments have a structure to detect multiple wavelengths by a plurality of half mirrors and a plurality of light-receiving elements. The half mirror-system spectroscopic instrument detects multiple wavelengths by splitting, at respective half mirrors, an incident light into a transmitted light and a reflected light and using the transmitted light as an incident light of a half mirror positioned in the next order. Therefore, compared to the above-described rotary plate-system spectroscopic instrument that detects multiple wavelengths by mechanically selecting a wavelength, multiple wavelengths can be speedily detected.